The users are able to search for a vehicle by typing a few characters of the vehicles "callsign".

My application is based around a sort of "event bus" within Backbone.. when a search occurs I send a message on the bus saying something like "does anyone match this?".. If a marker matches the search term it responds with a sort of "yes, I match!"..

My challenge arises when no-one matches, I get no response.. it feels a little hacky to "wait a little while" and check if a response has been recieved..

not a pattern, but design principle (that worked great for me in cases like that): expiration by timeout should be first class citizen in your application. Designed and carefully thought out from the scratch, easy to use by developers, easy to maintain by supporters (configuration, logging, audit trail...) and easy to understand by end users: "no response from vehicle in 5 minutes, timeout expired. Retry, cancel?"
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gnatNov 1 '12 at 7:54